Why is there a current of cold air from the bottom edge of the glass in the window?

This may be due to cold drafts

Cold drafts occur when air is cooled by a window. The cooled air is heavier than air in the rest of the room, so it sinks or "races" downward. The air is pushed across the floor by its own weight, giving the impression of a draft. There are three factors that affect drafts: the height, size and surface temperature of the window. The colder and higher the window, the faster the air will sink and the greater the "draft". NOTE! Underfloor heating will not prevent cold drafts by the window.

For better comfort and warmth, use NTech Passive or NTech 1.0 "Enova" which produces far less cold drafts.